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At this moment, our implementation only keeps the first and the
last keyframes for composite background color animation. This is
wrong, for example, we can having multiple keyframes like:
0%: { background-color: red }
10%: { background-color: green}
100% { background-color: blue}

This CL fixes the issue. The idea is to keep the offset which
we stored in CompositorKeyframeValues. In the above case, the
offsets would be [0, 0.1, 1].

Once we have that, here is how we do interpolation. Say that
the current progress is 0.4, then we know that it falls in to
the range of [0.1, 1], which means we need to interpolate from
green to blue. We need to adjust the progress (0.4) based on the
offsets. Basically scale [0.1, 1] to [0, 1], then the adjusted
progress should be (0.4-0.1) / (1-0.1). Layout tests are added
to ensure correctness.

Bug: 1153671
Change-Id: I6dffc82b5821fdf418f2b94331f567a239079730
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2597844
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841476}